Don’t try to bring a tiny tube of lip-moisturizing gel on board an airliner these days, or wear thin gel inserts to make your shoes more comfortable. And leave your cold and cough gel-caps at home. But, ladies and cross-dressing gents, no one will check if you wear your gel-filled bras right onto the plane.

Despite the fact that these hugely popular undergarments are owned by millions worldwide and have the capacity to carry enough liquid or gel explosives to make a terrorist smile, the Transportation Security Administration has not included them on the new list of items forbidden from carry-on baggage.

In the travel tips listed on the agency’s Web site — www.tsa.gov — mention is made of "gel-filled bras," but mostly in the context of those worn as prosthetics by breast-cancer survivors who have undergone mastectomies.

Such passengers are urged to pack their bras in their checked baggage, but also advised that those with "medical gel" prosthetics will be allowed through security checkpoints. The agency says it is "reaching out" to women’s medical associations to spread the word about the policy.

But asked about the tens of millions of women who wear silicone-gel and even plain water sacs in their bras to enhance their curves, Kayser said he was not aware of the wide popularity of that fashion trend.

He said these women should also stow their liquid- and gel-filled bras in their checked luggage. But Kayser said he knew of no efforts to publicize that fact or even include the bras on the banned-items list.

Asked whether — and how — airport screeners would differentiate between women wearing "medical gel" bras or those enhanced for cosmetic purposes, Kayser said women would be neither poked nor questioned about the contents of their undergarments. Instead, security would be ensured by other "levels" of security in airports.

"It’s not going to be something we’re going to ask about at checkpoints," Kayser said. "We have a lot of different ways to detect" dangerous substances.

While gel bras come in a variety of sizes and styles, it is not uncommon for one to carry 7 ounces or more of silicone or water in the cups — more than 170 times the amount of goop contained in a 0.04-ounce tube of lip gel.

Conceivably, a would-be evildoer could substitute explosive substances to make a "bra bomb" capable of blowing a hole in a plane’s fuselage. It would likely take a big box full of lip-gel tubes to do the same.

How serious a threat is this security loophole? Virtually all who have been arrested for conspiring to bomb airplanes have been men. But while the use of women as suicide bombers is rare, it is a growing phenomenon, according to a study released this week by the National Memorial Institute for the Prevention of Terrorism, in Oklahoma City.

Extremist "Muslim women are increasingly joining the global jihad," Farhana Ali wrote in a study in the 2006 edition of the institute’s Terrorism Annual Report. There is even a name for women jihadis — "mujahidaat."

One thing not in doubt is the popularity of gel-enhanced bras, which women — and thousands of cross-dressing men — have embraced to put curves and heft where little naturally exist, lingerie-industry experts say.

Beginning in Asia about a decade ago, the concept has swept across the globe, as women discovered the natural look and feel of what some call "outplants." Unlike surgically inserted breast implants, these carry neither a health risk nor a big price tag. An array of products is available — from small, removable gel pads to full-size ones that can "grow" a breast to DD size, or bigger.

"The last 10 years, the market has been soaring," said Camille Wiart, marketing executive with Lingerie-Americas, a major industry trade show.

Becky Davis, vice president of Lingerie Mart, the largest global wholesaler of brand-name lingerie, agrees.